The present invention relates to a reception determination method and system of a ray, and a radio wave propagation characteristic estimation method using them, and especially, to a reception determination method and system of a ray in radio wave propagation characteristic estimation by means of a geometric optical technique, and a radio wave propagation characteristic estimation method using them.
In order to efficiently arrange a base unit (base station) in a radio communication system, it becomes important that a radio wave propagation characteristic such as a propagation loss and delay spread can be predicted on a computer with good accuracy and at high speed. Usually, an arrangement of the base unit in the radio communication system is conducted by try and error so that a region where communication is desired would be a range where a codeless handset (terminal) can communicate. However, a method in which the base unit and codeless handset are installed in real environment to study the arrangement of the base unit by means of actual measurement generally requires tremendous costs. Accordingly, in case of suppressing an installation cost, a method in which a propagation model of the said environment is structured on a computer to study the arrangement of the base unit theoretically is used.
The range where the codeless handset can communicate is defined by comparing parameters such as intensity at which the codeless handset receives a radio wave radiated from the base unit, extent of distortion which a signal on a transmission line connecting the base unit to the codeless handset suffers, and intensity of an interference signal mixed into receivers of both base unit and codeless handset from a radio system other than the said radio system with a characteristic of the receivers. These parameters can be calculated from a radio wave propagation characteristic such as a propagation loss and delay spread of a transmission line between the base unit and the codeless handset of the said radio system, and a propagation loss between an external interference source and the base unit or the codeless handset of the said radio system. Accordingly, if the radio wave propagation characteristic such as a propagation loss and delay spread can be predicted on a computer with good accuracy and at high speed, an optimum arrangement of the base unit, which requires trial-and-error study in response to individual propagation environment, can be determined rapidly and correctly independent of actual measurement.
As a prior art for predicting the radio wave propagation characteristic by using the computer in this manner, there is a method in which a radio wave propagation simulator is used, and the radio wave propagation simulator forming the basis thereof can be generally classified broadly into a statistical technique and a deterministic technique. The statistical technique is a technique wherein a propagation loss estimation formula in which a distance, a frequency and so forth are arguments is used, and the parameters in the estimation formula are determined by means of a statistical technique such as a multivariate analysis based on a number of data obtained by the actual measurement of a propagation loss. Although this technique was conventionally used for application of a base station arrangement design or the like of outdoor mobile communication, it is generally an expensive technique since a number of actually measured data are required for determining the propagation loss estimation formula of the said environment with high accuracy. In addition, since this technique is one for statistically obtaining an average tendency of the radio wave propagation characteristic in a somewhat wide range, it is not appropriate for environment wherein the propagation characteristic tends to vary widely due to diversification of an indoor arrangement of an appliance, a wall or the like, and application for obtaining the local propagation characteristic in a comparatively narrow range even out of doors.
On the other hand, the deterministic technique is a technique for obtaining the propagation characteristic from a numerical analysis result of a Maxwell electromagnetic equation that is a fundamental equation of the radio wave propagation. Since this technique is based on a physical law, compared with the statistical technique, highly accurate prediction is possible. However, as described in a Non-Patent Publication 1, out of the deterministic techniques, in many techniques like an FDTD, a finite element method and so forth, a computational effort necessary for the analysis becomes huge as a wavelength becomes short compared with an analysis subject region of the propagation characteristic. Accordingly, a lot of computational resources are required for establishing high-speed and highly accurate prediction of the propagation characteristic in such a high frequency band.
Presently, in association with a growing demand for speeding-up of communication, development of a radio communication system which uses a higher frequency band is actively being conducted. Out of the deterministic techniques, there is a geometric optical technique (ray tracing) as a technique appropriate for an analysis of such a high frequency band. This technique has a feature that, compared with other deterministic techniques, highly accurate analysis of the high frequency band can be conducted at high speed.
This ray tracing is a technique in which a radio wave radiated from an antenna is represented by a bunch of a number of radio wave lines (rays), and assuming that each ray is propagated while repeating reflection and transmission geometrically-optically in an obstruction installed inside an analysis space, a locus thereof is calculated. A propagation loss and delay spread at a reception point can be obtained by combining electrical field strength and a propagation period of time of each ray which arrives at the reception point.
The ray tracing can be further classified broadly into an imaging method and a launching method because of a difference of a method of tracing a propagation path. One example of a radio wave propagation simulator using the imaging method is described in a Non-Patent Publication 2. As described in this publication, the imaging method is a technique for determining a reflection and transmission path of a ray, which connects transmission and reception points, while obtaining a mirror image of the transmission point for a reflection plane. Since the reflection and transmission path is obtained uniquely if positions of the transmission and reception points and a reflection and transmission barrier are determined, the imaging method is a technique for searching a rigorous propagation path of the ray.
On the other hand, the launching method is a technique in which, independently of a position of a reception point, a ray which was radiated from an antenna at a discrete angle interval and passed the vicinity of the reception point while repeating reflection, transmission or the like is regarded as a ray which reached the said reception point. Since, in the launching method, a solution of a propagation path of a ray, which connects the transmission and reception points, is not obtained rigorously different from the imaging method, but is obtained approximately, it has a feature that a time period required for the propagation path search can be significantly shortened.
FIG. 33 is a view explaining an outline of a flow of the processing of the launching method, and is also applied to the present invention. First, at a step 102, initial setting of a storage region in which structural information inside an observation region is stored, and a storage region in which propagation characteristic information of a reception point is stored is conducted, and calculation of a direction vector of a ray set radiated from a transmission point is conducted. Next, at a step 103, one ray is selected out of the ray set radiated from the transmission point, and a path of the ray after the selected ray was radiated from the transmission point is tracked (step 104). At this time, every time one sectional path is defined in process of path tracking processing, one section just after the definition is selected (step 112), and it is determined whether or not a ray passing through the selected path section is received at the reception point inside the observation region (step 107). These path tracking processing and reception determination processing continue until an end condition of the path tracking is satisfied (step 105), and the above processing is applied to all rays radiated from the transmission point (step 109), and a result is output and the processing is finished (step 110).
FIG. 34 is a view showing the particular processing in the step 107. In the step 107, first, one reception point inside the observation region is selected (step 114), and it is determined whether or not the ray passing on the path section selected at the step 112 is received at the reception point selected at the step 114 (step 115). If it is determined at the step 115 that it is not received, the processing promptly moves to the next process, and if it is determined that it is received, reception electrical field strength and delay time are calculated (step 117), and calculation results are stored in a storage region corresponding to the said reception point (step 118). The determination at the step 115 is performed for all reception points (step 116), and the reception determination processing is finished.
FIG. 35 is a view explaining the path tracking processing in case that an observation region 018, a transmission point 016, a reception point 017, and two objects 001 and 002 are provided. In FIG. 35, although, for simplification, the explanation of the processing will be made by restricting it to a two-dimensional plane, in actual, the processing can be certainly conducted within a three-dimensional space. One orientation of a ray to be radiated from the transmission point 016 is selected out of orientations 008-015 (In FIG. 35, an orientation 015 is selected.), and a ray is radiated. An object intersecting with a ray 003, end point of which is the transmission point 016, is searched from objects inside the observation region to obtain an intersection point 019, and in accordance with a geometrical optics theory, a reflected ray 005 and a transmitted ray 004 are generated. An object intersecting with the reflected ray 005 is searched from the objects inside the observation region to obtain intersection point 020, and a reflected ray 006 and a transmitted ray 007 are generated again. In this manner, the search of the object intersecting with the ray, the calculation of the intersection point, and the generation of the reflected ray and the transmitted ray are repeated, and the path tracking processing is finished at a time point when the reflected ray and the transmitted ray meet a tracking end condition.
For the tracking end condition, a case wherein the ray has reached the observation region 018, a case wherein the reflection number or the transmission number has reached a predetermined upper limit, a case wherein electrical field strength determined by a locus of the ray has been lower than a predetermined value and so forth are generally used. By means of such path tracking processing, in FIG. 35, for example, a path consisting of sections 003, 005 and 006, a path consisting of sections 003 and 004, and a path consisting of sections 003, 005 and 007 can be obtained.
One example of a prior art for determining an orientation of a ray to be radiated from the transmission point is described in a Non-Patent Publication 3. FIG. 36-FIG. 39 views explaining this prior art.
According to a method described in this publication, first, as shown in FIG. 36, a three-dimensional closed region of a regular icosahedron is provided around a transmission antenna 301. Next, after taking a plane forming the regular icosahedron, namely, a plane of a regular triangle, which is constructed of apexes 406, 407 and 408, as shown in FIG. 37, each side is divided at even intervals by using points 409, 410 and 411. By drawing line segments that are parallel with each side of the regular triangle constructed of the apexes 406, 407 and 408, and passes through divisional points, triangles similar to the original regular triangle are newly created inside. The above processing is applied to all planes constituting the regular icosahedron of FIG. 36, and if the apexes of each regular triangle are moved in a direction which connects a center of mass of the regular icosahedron to the apexes of the newly created regular triangle so that distances from the center of mass become equal to each other, a view like FIG. 38 for example can be obtained.
FIG. 38 is a view of a case in which one side of the regular triangle forming each plane of the regular icosahedron of FIG. 36 is halved. Rays to be radiated from the transmission antenna 501 positioned at the center of mass of the original regular icosahedron are radiated in each direction which connects the transmission antenna 501 to each apex of the polyhedron of FIG. 38. In FIG. 38, as one example, a ray 504 passing through an apex 502 is shown. The definite number of rays determined in this manner become a set of rays to be radiated from the transmission point.
In the vicinity of the ray created in accordance with the above-described method, a partial space can be defined, and by means of the partial space, a space around the transmission point 501 can be divided into mutually exclusive partial spaces. FIG. 39 is a view in which a partial space in the vicinity of the ray created in accordance with the above-described method is extracted. In the vicinity of the ray 504, a region 505 of a pyramid is defined, a cross-sectional shape thereof, which is taken by a plane perpendicular to the ray 504 being a hexagon and an apex thereof being the transmission point 501. By defining the partial space in the vicinity of the ray in this manner, the determination (step 107) on whether or not a certain reception point receives a ray passing through the vicinity thereof comes down to determination on whether or not the reception point is included in this partial space defined in the vicinity of the ray. In addition, although, following the above-described method, the partial space would be a regular six-sided pyramid or a regular five-sided pyramid, other than this, sometimes there is a case in which a triangular pyramid is used for the partial space.
Out of the ray tracing methods, compared with the launching method, a calculation load in the imaging method is much larger. Accordingly, some technologies for realizing speeding-up are devised, and one example of the conventional technologies is described in a Patent Publication 1 and a Patent Publication 2. However, even in case that these techniques for speeding-up are applied, generally an estimation speed of the imaging method is slower than that of the launching method. Also, an estimation accuracy of the imaging method is theoretically higher than that of the launching method, and however, since a structural model used in a simulation is often simplified to a certain extent, due to an effect of an error between the model and real environment, in most cases, the estimation accuracy of the imaging method is not so different from that of the launching method.
As mentioned above, in order to conduct an arrangement of the base unit of the radio communication system efficiently, it becomes important to be able to estimate a highly accurate radio wave propagation characteristic of a desired region with a high speed. Accordingly, if application to such use is considered, a technical approach for realizing the fast estimation by means of the launching method rather than the imaging method without deteriorating the estimation accuracy is important, and a prior art for realizing this is described in a Patent Publication 3.
In the processing of the launching method described in FIG. 33, as one of the processes, calculation load of which is large, there is the path tracking processing of the step 104. In the Patent Publication 3, a prior art for realizing this path tracking processing with a high speed is described. Since the path tracking processing accompanies the intersection determination of a ray and obstacles installed inside an analysis region, a calculation load increases in proportion to the total number of the obstacles. Also, in case that an upper limit for the total number of reflection and transmission is provided, and this would be an end condition of the path tracking, a calculation load of the path tracking increases if the upper limit for the total number of the reflection and transmission is raised. Further, with regard to the path tracking processing, the calculation load increases in proportion to the total number of the rays to be radiated from the transmission point.
Particularly, assuming that M is the total number of the obstacles installed inside the analysis region, N is the upper limit for the total number of the reflection and transmission, W is the total number of the rays to be radiated from the transmission point, and δ1 is calculation time required for one intersection determination, entire calculation time T1 required for the intersection determination of the obstacles and the ray is represented as follows:T1=MW(2N+1−1)δ1  (1)
In the launching method, generally, when the total number of the rays to be radiated from the transmission point or the total number of the reflection and transmission is increased, the estimation accuracy of the radio wave propagation characteristic is improved. On the other hand, generally, there are many obstacles inside the analysis space. Accordingly, in a case where a highly accurate estimation is conducted under real environment, a calculation load of the path tracking generally becomes large.
In the prior art described in the Patent Publication 3, the intersection determination of the obstacles and the ray is not conducted without variation, and step-by-step determination conditions in which a projected image is utilized and calculation loads are different from each other are used. According to this arrangement, the obstacles which do not intersect clearly can be excluded early by means of the determination conditions in which the calculation load is small, and as a result, reduction of Mδ1 in the equation (1) is possible, that is to say, the calculation time required for the intersection determination can be reduced.
[Patent Publication 1]
JP-P1996-008846A (U.S. Pat. No.3,256,085) (Page 4-Page 8, FIG. 1, FIG. 4-FIG. 24)
[Patent Publication 2]
JP-P1997-119955A (Page 4-Page 5, FIG. 8-FIG. 16)
[Patent Publication 3]
JP-P2002-107397A (Page 6-Page 8, FIG. 1-FIG. 5)
[Non-Patent Publication 1]
Eikichi Yamashita, “Fundamental Analysis Method of Electromagnetic Wave Questions”, pp.198, The Institute of Electronics, Information and Communication Engineers, 1987
[Non-Patent Publication 2]
J. W. MacKown and R. L. Hamilton Jr., “Ray Tracing as a Design Tool for Radio Networks” IEEE Network Mag, pp.27-30, November 1991
[Non-Patent Publication 3]
S. Y. Seidel and T. S. Rappaport, “Site-Specific Propagation Prediction for Wireless In-Building Personal Communication System Design” IEEE Trans Veh Technol, 43, 4, pp.879-891, 1994
In the processing of the launching method of FIG. 33, even though speeding-up of the path tracking is pursued to the maximum, there is a task that the calculation time required for the propagation estimation would not be equal to or less than a certain level. The reason thereof is that the calculation time required for the reception determination processing at the step 107 of FIG. 33 relatively becomes larger as the time required for the path tracking processing is shortened.
Since the reception determination processing (step 115) of FIG. 34 is surrounded by multiple loops consisting of the step 103 and the step 109, the step 104 and the step 105, and the step 114 and the step 116, any one of the total number of the rays to be radiated from the transmission point, the total number of the path sections, namely, the total number of the reflection and transmission, and the total number of the reception points is increased, it causes the increase of the entire calculation time required for the reception determination processing. Particularly, assuming that P is the total number of the reception points inside the analysis region, N is the upper limit for the total number of the reflection and transmission, W is the total number of the rays to be radiated from the transmission point, and δ2 is calculation time required for one reception determination, entire calculation time T2 required for the reception determination processing is represented as follows:T2=PW(2N+1−1)δ2  (2)
As a sum of the calculation time T1 required for the path tracking of the equation (1) and the calculation time T2 required for the reception determination processing of the equation (2), calculation time T3 required for the propagation estimation can be approximately represented as follows:T3=W(2N+1−1) (Mδ1+Pδ2)  (3)
As understood from the equation (3), as the speeding-up of the path tracking processing is undertaken to reduce Mδ1, the calculation time T3 required for the propagation estimation asymptotically moves closer to the entire calculation time T2 required for the reception determination processing, and finally, bottoms out at a value thereof. On the other hand, if the total number of the reception points is reduced to suppress the calculation time required for the reception determination, there is a task that it becomes difficult to look through the propagation characteristic of the entire observation region with a higher accuracy. A reason thereof is that, in case that the reception points are arranged inside the observation region in a lattice shape for the purpose of looking through the propagation characteristic of the entire observation region, when the number of the entire reception points is reduced, an individual lattice interval is broadened, and it becomes difficult to understand a tendency of the local propagation characteristic.